Figure 5

Determining the value of an overcrossing. When projection of a pair of oriented line segments results in an overcrossing, its value is determined by the right-hand rule involving the projection direction and directions of projected line segments. Here the projection direction is from the page to the reader. (a) The value of this overcrossing is +1 because the bottom line segment (u) is in the counterclockwise direction from the upper line segment (v). (b) The value of this overcrossing is -1 because the bottom line segment (u) is in the clockwise direction from the upper line segment (v).